This invention relates to zoom lens systems of the type comprising a front lens group having a negative focal length and a rear lens group having a positive focal length with the axial separation therebetween being variable for zooming purposes, and more particularly to an improvement of a zoom lens system of the type described in that, with a greatly increased image angle up to about 84.degree., it is possible to achieve minimization of the overall dimensions of the complete system without causing any decrease in the extent to which the corrections of various aberrations are maintained throughout the zooming range irrespective of the great increase in the photographable range extended toward the wide angle side.
As shown in FIG. 1, a zoom system which may be conveniently considered as comprising a negative front lens I and a positive rear lens II, both of which are moved axially but in differential relation to each other to effect variation of the focal length of the overall lens system is, because of its being of inverted telephoto type, advantageous in providing an increased extent of image angle.
It is known to provide such type zoom lens as disclosed, for example, in U.S. Pat. Nos. 3,143,590 and 3,848,969. As the maximum axial separation between the front and rear lenses I and II occurs at the wide angle setting for the maximum image angle, however, there is a disadvantage to this zoom system such that an increase in the diameter of the front lens I must be made for the extended range of image angles toward the maximum. This problem becomes serious in designing a zoom lens to increase the image angle thereof to a great extent. If the provision for increasing the maximum image angle is forced to be compatible with severe limitation of the overall dimensions of the zoom lens to a minimum, the barrel type distortion in the wide angle position is rapidly increased and its satisfactory correction is made difficult by any means.